This disclosure claims the benefit of priority under 35 U.S.C. §119 to European Patent Application No. EP 10003522.9, which was filed on Mar. 31, 2010. The entire content of the cross-referenced European application is incorporated by reference herein.
1. Technical Field
The invention relates to a feedback apparatus and a feedback method for controlling a servo motor.
2. Related Art
Feedback systems are used for controlling electric servo motors. These systems perform the task of providing the actual data required for controlling speed and angular position.
Known feedback systems use rotary angle sensors (encoders) to measure angular position in absolute or incremental terms. Rotary angle sensors currently in use have a graduation, which is scanned optically, magnetically, inductively or capacitively. Each scan produces a sine signal and a cosine signal, which allow an interpolation of the graduation. With rotary angle sensors of this type, graduations having 1024 or 2048 periods per revolution are customary. Interpolation is then possible up to 14 bits per period. This results in angular resolutions for such rotary angle sensors of up to 25 bits per revolution.
The accuracy of rotary angle sensors that measure with such high resolution is dependent on the precision of the graduation. To control the rotary speed of the servo motor, the actual rotary speed is determined by determining the difference between rotary angle measurements. Errors in the differential and integral linearity of the graduation therefore lead to an increasing extent to errors in the actual rotary speed values provided by the feedback system. This can be disadvantageous particularly when the controller is adjusted to a high compliance level, i.e., it reacts very rapidly to changes in the actual value. Even if the servo motor is itself running at the set rotary speed, apparent fluctuations in rotary speed simulated by the nonlinearity of rotary angle measurement will be corrected by accelerations and decelerations of the servo motor, which can result in substantial losses in energy, particularly when larger masses are being driven. In conventional rotary angle sensors, this disadvantage can be diminished only by improving the differential and integral linearity of the graduation, which substantially increases the cost of producing the graduation.
State-of-the-art feedback systems that use rotary angle sensors of this type operate reliably at rotary speeds of up to about 12,000 revolutions per minute (rpm). At lower rotary speed ranges, for example at rotary speeds of <5 rpm, determining the actual rotary speed by determining the difference between angular position values becomes imprecise, because the angular resolution of the interpolation of the graduation periods is diminished.
It is further known to use rotary speed sensors for measuring the rotary speed of rotating objects. Such rotary speed sensors are preferably embodied as optical gyroscopes, for example, as fiber optic gyroscopes (fiber optic gyros) or as ring lasers (ring laser gyros). With optical gyroscopes of this type, the difference in time delay between a laser beam running in the direction of rotation and a laser beam running counter to the direction of rotation is used to determine rotary speed. As is described, for example, in EP 585 954 B1, the two laser beams are placed in interference, wherein the displacement of the interference pattern provides the measurement of rotary speed. Rotary speed sensors of this type, and particularly optical gyroscopes, permit a highly precise measurement of rotary speed. For example, rotary speed can be measured at a resolution of <10−3 rpm. By integrating the rotary steed measurements over time, rotary angle position can be determined with corresponding accuracy.
One problem with optical gyroscopes is that their measurements are dependent on the inertial system in which the gyroscope is placed. Therefore, rotary speed measurements are always influenced by the earth's rotation, and in many applications these measurements can also be influenced by a possible rotation of the machine or the apparatus in which the gyroscope is placed. These factors must be corrected when determining rotary speed measurement. A further problem is that such gyroscopes frequently have substantial drift.
Measuring rotary speed over a displacement of the optical interference fringe pattern, in other words over a change in light intensity, results in measurement at higher rotary speeds becoming difficult. The measuring range for such optical gyroscopes is therefore limited to a maximum of about 100 rpm.